1. Field of the Invention
The invention is directed to a method for producing electrically conducting composites from predominantly nonconductive or only poorly conductive substances, in which the conductivity is achieved by the addition of unusually small proportions of highly conductive materials. A special feature of the composites according to the invention is the favorable spatial distribution of the added highly conductive materials which is achieved.
2. Discussion of the Prior Art
The production of conductive composites from predominantly nonconductive materials by adding conductive substances is attempted for many areas of application. These include conducting, and accordingly shielding, plastic housings for electronic devices or battery electrodes, for instance, which contain poorly conducting materials such as manganese dioxide as electrochemically active components.
There have been many studies concerned with the dependence of conductivity on composites containing conductive and nonconductive substances. The most important insight of these studies is that an increase in the conductivity of the composite is in no way proportional to the addition of electron-conducting components. Rather only after exceeding a threshold value of the proportion of conductive components, which is usually quite high, does the conductivity of the composite also increase (e.g., see K.-J. Euler, R. Kirchhof, H. Metzendorf, J Power Sources, 5 (1980) p. 255 and K. Aoki, J. Electroanal. Chem. 292 (1990), p. 53).
The reason for this effect is that substantial conductivity of the composite only occurs when the conducting component is so closely packed that there occur elongated "chains" or "networks" of conductive particles which provide for conductivity over great distances. Assuming uniformly shaped particles of the conductive and nonconductive component, the conductivity of mixtures of conductive and nonconductive particles can be calculated with the aid of the "percolation theory". Qualitatively speaking, the likelihood of the formation of elongated chains or networks of the conductive component is sufficiently high only with very high proportions of this component.
There are a number of methods within the prior art which basically enable a conductive coating of nonconductors. First, there are those methods which apply chiefly metal layers to nonconductors in vacuo by evaporation or sputtering (cathode sputtering). Numerous metals and other materials can also be deposited in thin layers on nonconductors by chemical vapor phase deposition (CVD).
However, these methods are poorly suited for coating powders, fibers and the like finely particulate materials, since it would be necessary to keep these finely particulate materials in constant motion in the gas phase or under vacuum in order to cover them on all sides. On the other hand, coating processes using solutions, e.g. the "electroless" metallization process (C. R. Shipley Jr., Plating and Surface Finishing, 71 (1984) 92) are considerably more manageable for coating large quantities of small particles. Deposition of copper, nickel, gold and silver is particularly successful with such electroless processes.